Traffic collision avoidance in wireless communication systems

ABSTRACT

A low cost, robust, wireless sensor that provides an extended period of operability without maintenance is described. The wireless sensors are configured to communicate with a base unit or repeater. When the sensor unit detects an anomalous ambient condition (e.g., smoke, fire, water, etc.) the sensor communicates with the base unit and provides data regarding the anomalous condition. The sensor unit receives instructions to change operating parameters and/or control external devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/168,876, filed Jan. 30, 2014, and entitled “WIRELESS TRANSCEIVER”, which is a continuation of U.S. patent application Ser. No. 12/905,248, filed Oct. 15, 2010, and entitled, “WIRELESS TRANSCEIVER,” which is a continuation of U.S. patent application Ser. No. 12/182,079, filed Jul. 29, 2008, and entitled “WIRELESS TRANSCEIVER,” now U.S. Pat. No. 7,817,031, which is a divisional of U.S. patent application Ser. No. 11/562,313, filed Nov. 21, 2006, and entitled “WIRELESS TRANSCEIVER,” now U.S. Pat. No. 7,411,494, which is a continuation of U.S. patent application Ser. No. 10/856,231, filed May 27, 2004, and entitled “WIRELESS TRANSCEIVER,” now U.S. Pat. No. 7,142,107. The entire disclosures of the above applications are hereby incorporated by reference, for all purposes, as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless sensor unit system providing bi-directional communication between a sensor (e.g., smoke sensor, fire sensor, temperature sensor, water, etc.) and a repeater or base unit in a building protection system.

2. Description of the Related Art

Maintaining and protecting a building or complex is difficult and costly. Some conditions, such as fires, gas leaks, etc. are a danger to the occupants and the structure. Other malfunctions, such as water leaks in roofs, plumbing, etc. are not necessarily dangerous for the occupants, but can nevertheless cause considerable damage. In many cases, an adverse ambient condition such as water leakage, fire, etc. is not detected in the early stages when the damage and/or danger is relatively small. Sensors can be used to detect such adverse ambient conditions, but sensors present their own set of problems. For example, adding sensors, such as, for example, smoke detectors, water sensors, and the like in an existing structure can be prohibitively expensive due to the cost of installing wiring between the remote sensors and a centralized monitoring device used to monitor the sensors. Adding wiring to provide power to the sensors further increases the cost. Moreover, with regard to fire sensors, most fire departments will not allow automatic notification of the fire department based on the data from a smoke detector alone. Most fire departments require that a specific temperature rate-of-rise be detected before an automatic fire alarm system can notify the fire department. Unfortunately, detecting fire by temperature rate-of-rise generally means that the fire is not detected until it is too late to prevent major damage.

SUMMARY

The present invention solves these and other problems by providing a relatively low cost, robust, wireless sensor system that provides an extended period of operability without maintenance. The system includes one or more intelligent sensor units and a base unit that can communicate with the sensor units. When one or more of the sensor units detects an anomalous condition (e.g., smoke, fire, water, etc.) the sensor unit communicates with the base unit and provides data regarding the anomalous condition. The base unit can contact a supervisor or other responsible person by a plurality of techniques, such as, telephone, pager, cellular telephone, Internet (and/or local area network), etc. In one embodiment, one or more wireless repeaters are used between the sensor units and the base unit to extend the range of the system and to allow the base unit to communicate with a larger number of sensors.

In one embodiment, the sensor system includes a number of sensor units located throughout a building that sense conditions and report anomalous results back to a central reporting station. The sensor units measure conditions that might indicate a fire, water leak, etc. The sensor units report the measured data to the base unit whenever the sensor unit determines that the measured data is sufficiently anomalous to be reported. The base unit can notify a responsible person such as, for example a building manager, building owner, private security service, etc. In one embodiment, the sensor units do not send an alarm signal to the central location. Rather, the sensors send quantitative measured data (e.g., smoke density, temperature rate of rise, etc.) to the central reporting station.

In one embodiment, the sensor system includes a battery-operated sensor unit that detects a condition, such as, for example, smoke, temperature, humidity, moisture, water, water temperature, carbon monoxide, natural gas, propane gas, other flammable gases, radon, poison gasses, etc. The sensor unit is placed in a building, apartment, office, residence, etc. In order to conserve battery power, the sensor is normally placed in a low-power mode. In one embodiment, while in the low power mode, the sensor unit takes regular sensor readings and evaluates the readings to determine if an anomalous condition exists. If an anomalous condition is detected, then the sensor unit “wakes up” and begins communicating with the base unit or with a repeater. At programmed intervals, the sensor also “wakes up” and sends status information to the base unit (or repeater) and then listens for commands for a period of time.

In one embodiment, the sensor unit is bi-directional and configured to receive instructions from the central reporting station (or repeater). Thus, for example, the central reporting station can instruct the sensor to: perform additional measurements; go to a standby mode; wake up; report battery status; change wake-up interval; run self-diagnostics and report results; etc. In one embodiment, the sensor unit also includes a tamper switch. When tampering with the sensor is detected, the sensor reports such tampering to the base unit. In one embodiment, the sensor reports its general health and status to the central reporting station on a regular basis (e.g., results of self-diagnostics, battery health, etc.).

In one embodiment, the sensor unit provides two wake-up modes, a first wake-up mode for taking measurements (and reporting such measurements if deemed necessary), and a second wake-up mode for listening for commands from the central reporting station. The two wake-up modes, or combinations thereof, can occur at different intervals.

In one embodiment, the sensor units use spread-spectrum techniques to communicate with the base unit and/or the repeater units. In one embodiment, the sensor units use frequency-hopping spread-spectrum. In one embodiment, each sensor unit has an Identification code (ID) and the sensor units attaches its ID to outgoing communication packets. In one embodiment, when receiving wireless data, each sensor unit ignores data that is addressed to other sensor units.

The repeater unit is configured to relay communications traffic between a number of sensor units and the base unit. The repeater units typically operate in an environment with several other repeater units and thus each repeater unit contains a database (e.g., a lookup table) of sensor IDs. During normal operation, the repeater only communicates with designated wireless sensor units whose IDs appears in the repeater's database. In one embodiment, the repeater is battery-operated and conserves power by maintaining an internal schedule of when its designated sensors are expected to transmit and going to a low-power mode when none of its designated sensor units is scheduled to transmit. In one embodiment, the repeater uses spread-spectrum to communicate with the base unit and the sensor units. In one embodiment, the repeater uses frequency-hopping spread-spectrum to communicate with the base unit and the sensor units. In one embodiment, each repeater unit has an ID and the repeater unit attaches its ID to outgoing communication packets that originate in the repeater unit. In one embodiment, each repeater unit ignores data that is addressed to other repeater units or to sensor units not serviced by the repeater.

In one embodiment, the repeater is configured to provide bi-directional communication between one or more sensors and a base unit. In one embodiment, the repeater is configured to receive instructions from the central reporting station (or repeater). Thus, for example, the central reporting station can instruct the repeater to: send commands to one or more sensors; go to standby mode; “wake up”; report battery status; change wake-up interval; run self-diagnostics and report results; etc.

The base unit is configured to receive measured sensor data from a number of sensor units. In one embodiment, the sensor information is relayed through the repeater units. The base unit also sends commands to the repeater units and/or sensor units. In one embodiment, the base unit includes a diskless PC that runs off of a CD-ROM, flash memory, DVD, or other read-only device, etc. When the base unit receives data from a wireless sensor indicating that there may be an emergency condition (e.g., a fire or excess smoke, temperature, water, flammable gas, etc.) the base unit will attempt to notify a responsible party (e.g., a building manager) by several communication channels (e.g., telephone, Internet, pager, cell phone, etc.). In one embodiment, the base unit sends instructions to place the wireless sensor in an alert mode (inhibiting the wireless sensor's low-power mode). In one embodiment, the base unit sends instructions to activate one or more additional sensors near the first sensor.

In one embodiment, the base unit maintains a database of the health, battery status, signal strength, and current operating status of all of the sensor units and repeater units in the wireless sensor system. In one embodiment, the base unit automatically performs routine maintenance by sending commands to each sensor to run a self-diagnostic and report the results. The bases unit collects such diagnostic results. In one embodiment, the base unit sends instructions to each sensor telling the sensor how long to wait between “wakeup” intervals. In one embodiment, the base unit schedules different wakeup intervals to different sensors based on the sensor's health, battery health, location, etc. In one embodiment, the base unit sends instructions to repeaters to route sensor information around a failed repeater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sensor system that includes a plurality of sensor units that communicate with a base unit through a number of repeater units.

FIG. 2 is a block diagram of a sensor unit.

FIG. 3 is a block diagram of a repeater unit.

FIG. 4 is a block diagram of the base unit.

FIG. 5 shows one embodiment a network communication packet used by the sensor units, repeater units, and the base unit.

FIG. 6 is a flowchart showing operation of a sensor unit that provides relatively continuous monitoring.

FIG. 7 is a flowchart showing operation of a sensor unit that provides periodic monitoring.

FIG. 8 shows how the sensor system can be used to detected water leaks.

DETAILED DESCRIPTION

FIG. 1 shows an sensor system 100 that includes a plurality of sensor units 102-106 that communicate with a base unit 112 through a number of repeater units 110-111. The sensor units 102-106 are located throughout a building 101. Sensor units 102-104 communicate with the repeater 110. Sensor units 105-105 communicate with the repeater 111. The repeaters 110-111 communicate with the base unit 112. The base unit 112 communicates with a monitoring computer system 113 through a computer network connection such as, for example, Ethernet, wireless Ethernet, firewire port, Universal Serial Bus (USB) port, bluetooth, etc. The computer system 113 contacts a building manager, maintenance service, alarm service, or other responsible personnel 120 using one or more of several communication systems such as, for example, telephone 121, pager 122, cellular telephone 123 (e.g., direct contact, voicemail, text, etc.), and/or through the Internet and/or local area network 124 (e.g., through email, instant messaging, network communications, etc.). In one embodiment, multiple base units 112 are provided to the monitoring computer 113. In one embodiment, the monitoring computer 113 is provided to more than one compute monitor, thus allowing more data to be displayed than can conveniently be displayed on a single monitor. In one embodiment, the monitoring computer 113 is provided to multiple monitors located in different locations, thus allowing the data form the monitoring computer 113 to be displayed in multiple locations.

The sensor units 102-106 include sensors to measure conditions, such as, for example, smoke, temperature, moisture, water, water temperature, humidity, carbon monoxide, natural gas, propane gas, security alarms, intrusion alarms (e.g., open doors, broken windows, open windows, and the like), other flammable gases, radon, poison gasses, etc. Different sensor units can be configured with different sensors or with combinations of sensors. Thus, for example, in one installation the sensor units 102 and 104 could be configured with smoke and/or temperature sensors while the sensor unit 103 could be configured with a humidity sensor.

The discussion that follows generally refers to the sensor unit 102 as an example of a sensor unit, with the understanding that the description of the sensor unit 102 can be applied to many sensor units. Similarly, the discussion generally refers to the repeater 110 by way of example, and not limitation. It will also be understood by one of ordinary skill in the art that repeaters are useful for extending the range of the sensor units 102-106 but are not required in all embodiments. Thus, for example in one embodiment, one or more of the sensor units 102-106 can communicate directly with the base unit 112 without going through a repeater. It will also be understood by one of ordinary skill in the art that FIG. 1 shows only five sensor units (102-106) and two repeater units (110-111) for purposes of illustration and not by way of limitation. An installation in a large apartment building or complex would typically involve many sensor units and repeater units. Moreover, one of ordinary skill in the art will recognize that one repeater unit can service relatively many sensor units. In one embodiment, the sensor units 102 can communicate directly with the base unit 112 without going through a repeater 111.

When the sensor unit 102 detects an anomalous condition (e.g., smoke, fire, water, etc.) the sensor unit communicates with the appropriate repeater unit 110 and provides data regarding the anomalous condition. The repeater unit 110 forwards the data to the base unit 112, and the base unit 112 forwards the information to the computer 113. The computer 113 evaluates the data and takes appropriate action. If the computer 113 determines that the condition is an emergency (e.g., fire, smoke, large quantities of water), then the computer 113 contacts the appropriate personnel 120. If the computer 113 determines that the situation warrants reporting, but is not an emergency, then the computer 113 logs the data for later reporting. In this way, the sensor system 100 can monitor the conditions in and around the building 101.

In one embodiment, the sensor unit 102 has an internal power source (e.g., battery, solar cell, fuel cell, etc.). In order to conserve power, the sensor unit 102 is normally placed in a low-power mode. In one embodiment, using sensors that require relatively little power, while in the low power mode the sensor unit 102 takes regular sensor readings and evaluates the readings to determine if an anomalous condition exists. In one embodiment, using sensors that require relatively more power, while in the low power mode the sensor unit 102 takes and evaluates sensor readings at periodic intervals. If an anomalous condition is detected, then the sensor unit 102 “wakes up” and begins communicating with the base unit 112 through the repeater 110. At programmed intervals, the sensor unit 102 also “wakes up” and sends status information (e.g., power levels, self diagnostic information, etc.) to the base unit (or repeater) and then listens for commands for a period of time. In one embodiment, the sensor unit 102 also includes a tamper detector. When tampering with the sensor unit 102 is detected, the sensor unit 102 reports such tampering to the base unit 112.

In one embodiment, the sensor unit 102 provides bi-directional communication and is configured to receive data and/or instructions from the base unit 112. Thus, for example, the base unit 112 can instruct the sensor unit 102 to perform additional measurements, to go to a standby mode, to wake up, to report battery status, to change wake-up interval, to run self-diagnostics and report results, etc. In one embodiment, the sensor unit 102 reports its general health and status on a regular basis (e.g., results of self-diagnostics, battery health, etc.).

In one embodiment, the sensor unit 102 provides two wake-up modes, a first wake-up mode for taking measurements (and reporting such measurements if deemed necessary), and a second wake-up mode for listening for commands from the central reporting station. The two wake-up modes, or combinations thereof, can occur at different intervals.

In one embodiment, the sensor unit 102 use spread-spectrum techniques to communicate with the repeater unit 110. In one embodiment, the sensor unit 102 use frequency-hopping spread-spectrum. In one embodiment, the sensor unit 102 has an address or identification (ID) code that distinguishes the sensor unit 102 from the other sensor units. The sensor unit 102 attaches its ID to outgoing communication packets so that transmissions from the sensor unit 102 can be identified by the repeater 110. The repeater 110 attaches the ID of the sensor unit 102 to data and/or instructions that are transmitted to the sensor unit 102. In one embodiment, the sensor unit 102 ignores data and/or instructions that are addressed to other sensor units.

In one embodiment, the sensor unit 102 includes a reset function. In one embodiment, the reset function is activated by the reset switch 208. In one embodiment, the reset function is active for a prescribed interval of time. During the reset interval, the transceiver 203 is in a receiving mode and can receive the identification code from an external programmer. In one embodiment, the external programmer wirelessly transmits a desired identification code. In one embodiment, the identification code is programmed by an external programmer that is connected to the sensor unit 102 through an electrical connector. In one embodiment, the electrical connection to the sensor unit 102 is provided by sending modulated control signals (power line carrier signals) through a connector used to connect the power source 206. In one embodiment, the external programmer provides power and control signals. In one embodiment, the external programmer also programs the type of sensor(s) installed in the sensor unit. In one embodiment, the identification code includes an area code (e.g., apartment number, zone number, floor number, etc.) and a unit number (e.g., unit 1, 2, 3, etc.).

In one embodiment, the sensor communicates with the repeater on the 900 MHz band. This band provides good transmission through walls and other obstacles normally found in and around a building structure. In one embodiment, the sensor communicates with the repeater on bands above and/or below the 900 MHz band. In one embodiment, the sensor, repeater, and/or base unit listen to a radio frequency channel before transmitting on that channel or before beginning transmission. If the channel is in use, (e.g., by another devise such as another repeater, a cordless telephone, etc.) then the sensor, repeater, and/or base unit changes to a different channel. In one embodiment, the sensor, repeater, and/or base unit coordinate frequency hopping by listening to radio frequency channels for interference and using an algorithm to select a next channel for transmission that avoids the interference. Thus, for example, in one embodiment, if a sensor senses a dangerous condition and goes into a continuous transmission mode, the sensor will test (e.g., listen to) the channel before transmission to avoid channels that are blocked, in use, or jammed. In one embodiment, the sensor continues to transmit data until it receives an acknowledgement from the base unit that the message has been received. In one embodiment, the sensor transmits data having a normal priority (e.g., status information) and does not look for an acknowledgement, and the sensor transmits data having elevated priority (e.g., excess smoke, temperature, etc.) until an acknowledgement is received.

The repeater unit 110 is configured to relay communications traffic between the sensor 102 (and, similarly, the sensor units 103-104) and the base unit 112. The repeater unit 110 typically operates in an environment with several other repeater units (such as the repeater unit 111 in FIG. 1) and thus the repeater unit 110 contains a database (e.g., a lookup table) of sensor unit IDs. In FIG. 1, the repeater 110 has database entries for the Ids of the sensors 102-104, and thus the sensor 110 will only communicate with sensor units 102-104. In one embodiment, the repeater 110 has an internal power source (e.g., battery, solar cell, fuel cell, etc.) and conserves power by maintaining an internal schedule of when the sensor units 102-104 are expected to transmit. In one embodiment, the repeater unit 110 goes to a low-power mode when none of its designated sensor units is scheduled to transmit. In one embodiment, the repeater 110 uses spread-spectrum techniques to communicate with the base unit 112 and with the sensor units 102-104. In one embodiment, the repeater 110 uses frequency-hopping spread-spectrum to communicate with the base unit 112 and the sensor units 102-104. In one embodiment, the repeater unit 110 has an address or identification (ID) code and the repeater unit 110 attaches its address to outgoing communication packets that originate in the repeater (that is, packets that are not being forwarded). In one embodiment, the repeater unit 110 ignores data and/or instructions that are addressed to other repeater units or to sensor units not serviced by the repeater 110.

In one embodiment, the base unit 112 communicates with the sensor unit 102 by transmitting a communication packet addressed to the sensor unit 102. The repeaters 110 and 111 both receive the communication packet addressed to the sensor unit 102. The repeater unit 111 ignores the communication packet addressed to the sensor unit 102. The repeater unit 110 transmits the communication packet addressed to the sensor unit 102 to the sensor unit 102. In one embodiment, the sensor unit 102, the repeater unit 110, and the base unit 112 communicate using Frequency-Hopping Spread Spectrum (FHSS), also known as channel-hopping.

Frequency-hopping wireless systems offer the advantage of avoiding other interfering signals and avoiding collisions. Moreover, there are regulatory advantages given to systems that do not transmit continuously at one frequency. Channel-hopping transmitters change frequencies after a period of continuous transmission, or when interference is encountered. These systems may have higher transmit power and relaxed limitations on in-band spurs. FCC regulations limit transmission time on one channel to 400 milliseconds (averaged over 10-20 seconds depending on channel bandwidth) before the transmitter must change frequency. There is a minimum frequency step when changing channels to resume transmission. If there are 25 to 49 frequency channels, regulations allow effective radiated power of 24 dBm, spurs must be −20 dBc, and harmonics must be −41.2 dBc. With 50 or more channels, regulations allow effective radiated power to be up to 30 dBm.

In one embodiment, the sensor unit 102, the repeater unit 110, and the base unit 112 communicate using FHSS wherein the frequency hopping of the sensor unit 102, the repeater unit 110, and the base unit 112 are not synchronized such that at any given moment, the sensor unit 102 and the repeater unit 110 are on different channels. In such a system, the base unit 112 communicates with the sensor unit 102 using the hop frequencies synchronized to the repeater unit 110 rather than the sensor unit 102. The repeater unit 110 then forwards the data to the sensor unit using hop frequencies synchronized to the sensor unit 102. Such a system largely avoids collisions between the transmissions by the base unit 112 and the repeater unit 110.

In one embodiment, the sensor units 102-106 all use FHSS and the sensor units 102-106 are not synchronized. Thus, at any given moment, it is unlikely that any two or more of the sensor units 102-106 will transmit on the same frequency. In this manner, collisions are largely avoided. In one embodiment, collisions are not detected but are tolerated by the system 100. If a collisions does occur, data lost due to the collision is effectively re-transmitted the next time the sensor units transmit sensor data. When the sensor units 102-106 and repeater units 110-111 operate in asynchronous mode, then a second collision is highly unlikely because the units causing the collisions have hopped to different channels. In one embodiment, the sensor units 102-106, repeater units 110-110, and the base unit 112 use the same hop rate. In one embodiment, the sensor units 102-106, repeater units 110-110, and the base unit 112 use the same pseudo-random algorithm to control channel hopping, but with different starting seeds. In one embodiment, the starting seed for the hop algorithm is calculated from the ID of the sensor units 102-106, repeater units 110-110, or the base unit 112.

In an alternative embodiment, the base unit communicates with the sensor unit 102 by sending a communication packet addressed to the repeater unit 110, where the packet sent to the repeater unit 110 includes the address of the sensor unit 102. The repeater unit 102 extracts the address of the sensor unit 102 from the packet and creates and transmits a packet addressed to the sensor unit 102.

In one embodiment, the repeater unit 110 is configured to provide bi-directional communication between its sensors and the base unit 112. In one embodiment, the repeater 110 is configured to receive instructions from the base unit 110. Thus, for example, the base unit 112 can instruct the repeater to: send commands to one or more sensors; go to standby mode; “wake up”; report battery status; change wake-up interval; run self-diagnostics and report results; etc.

The base unit 112 is configured to receive measured sensor data from a number of sensor units either directly, or through the repeaters 110-111. The base unit 112 also sends commands to the repeater units 110-111 and/or to the sensor units 110-111. In one embodiment, the base unit 112 communicates with a diskless computer 113 that runs off of a CD-ROM. When the base unit 112 receives data from a sensor unit 102-111 indicating that there may be an emergency condition (e.g., a fire or excess smoke, temperature, water, etc.) the computer 113 will attempt to notify the responsible party 120.

In one embodiment, the computer 112 maintains a database of the health, power status (e.g., battery charge), and current operating status of all of the sensor units 102-106 and the repeater units 110-111. In one embodiment, the computer 113 automatically performs routine maintenance by sending commands to each sensor unit 102-106 to run a self-diagnostic and report the results. The computer 113 collects and logs such diagnostic results. In one embodiment, the computer 113 sends instructions to each sensor unit 102-106 telling the sensor how long to wait between “wakeup” intervals. In one embodiment, the computer 113 schedules different wakeup intervals to different sensor unit 102-106 based on the sensor unit's health, power status, location, etc. In one embodiment, the computer 113 schedules different wakeup intervals to different sensor unit 102-106 based on the type of data and urgency of the data collected by the sensor unit (e.g., sensor units that have smoke and/or temperature sensors produce data that should be checked relatively more often than sensor units that have humidity or moisture sensors). In one embodiment, the base unit sends instructions to repeaters to route sensor information around a failed repeater.

In one embodiment, the computer 113 produces a display that tells maintenance personnel which sensor units 102-106 need repair or maintenance. In one embodiment, the computer 113 maintains a list showing the status and/or location of each sensor according to the ID of each sensor.

In one embodiment, the sensor units 102-106 and/or the repeater units 110-111 measure the signal strength of the wireless signals received (e.g., the sensor unit 102 measures the signal strength of the signals received from the repeater unit 110, the repeater unit 110 measures the signal strength received from the sensor unit 102 and/or the base unit 112). The sensor units 102-106 and/or the repeater units 110-111 report such signal strength measurement back to the computer 113. The computer 113 evaluates the signal strength measurements to ascertain the health and robustness of the sensor system 100. In one embodiment, the computer 113 uses the signal strength information to re-route wireless communications traffic in the sensor system 100. Thus, for example, if the repeater unit 110 goes offline or is having difficulty communicating with the sensor unit 102, the computer 113 can send instructions to the repeater unit 111 to add the ID of the sensor unit 102 to the database of the repeater unit 111 (and similarly, send instructions to the repeater unit 110 to remove the ID of the sensor unit 102), thereby routing the traffic for the sensor unit 102 through the router unit 111 instead of the router unit 110.

FIG. 2 is a block diagram of the sensor unit 102. In the sensor unit 102, one or more sensors 201 and a transceiver 203 are provided to a controller 202. The controller 202 typically provides power, data, and control information to the sensor(s) 201 and the transceiver 202. A power source 206 is provided to the controller 202. An optional tamper sensor 205 is also provided to the controller 202. A reset device (e.g., a switch) 208 is proved to the controller 202. In one embodiment, an optional audio output device 209 is provided. In one embodiment, the sensor 201 is configured as a plug-in module that can be replaced relatively easily.

In one embodiment, the transceiver 203 is based on a TRF 6901 transceiver chip from Texas Instruments, Inc. In one embodiment, the controller 202 is a conventional programmable microcontroller. In one embodiment, the controller 202 is based on a Field Programmable Gate Array (FPGA), such as, for example, provided by Xilinx Corp. In one embodiment, the sensor 201 includes an optoelectric smoke sensor with a smoke chamber. In one embodiment, the sensor 201 includes a thermistor. In one embodiment, the sensor 201 includes a humidity sensor. In one embodiment, the sensor 201 includes an sensor, such as, for example, a water level sensor, a water temperature sensor, a carbon monoxide sensor, a moisture sensor, a water flow sensor, natural gas sensor, propane sensor, etc.

The controller 202 receives sensor data from the sensor(s) 201. Some sensors 201 produce digital data. However, for many types of sensors 201, the sensor data is analog data. Analog sensor data is converted to digital format by the controller 202. In one embodiment, the controller evaluates the data received from the sensor(s) 201 and determines whether the data is to be transmitted to the base unit 112. The sensor unit 102 generally conserves power by not transmitting data that falls within a normal range. In one embodiment, the controller 202 evaluates the sensor data by comparing the data value to a threshold value (e.g., a high threshold, a low threshold, or a high-low threshold). If the data is outside the threshold (e.g., above a high threshold, below a low threshold, outside an inner range threshold, or inside an outer range threshold), then the data is deemed to be anomalous and is transmitted to the base unit 112. In one embodiment, the data threshold is programmed into the controller 202. In one embodiment, the data threshold is programmed by the base unit 112 by sending instructions to the controller 202. In one embodiment, the controller 202 obtains sensor data and transmits the data when commanded by the computer 113.

In one embodiment, the tamper sensor 205 is configured as a switch that detects removal of or tampering with the sensor unit 102.

FIG. 3 is a block diagram of the repeater unit 110. In the repeater unit 110, a first transceiver 302 and a second transceiver 305 are provided to a controller 303. The controller 303 typically provides power, data, and control information to the transceivers 302, 304. A power source 306 is provided to the controller 303. An optional tamper sensor (not shown) is also provided to the controller 303.

When relaying sensor data to the base unit 112, the controller 303 receives data from the first transceiver 303 and provides the data to the second transceiver 304. When relaying instructions from the base unit 112 to a sensor unit, the controller 303 receives data from the second transceiver 304 and provides the data to the first transceiver 302. In one embodiment, the controller 303 conserves power by powering-down the transceivers 302, 304 during periods when the controller 303 is not expecting data. The controller 303 also monitors the power source 306 and provides status information, such as, for example, self-diagnostic information and/or information about the health of the power source 306, to the base unit 112. In one embodiment, the controller 303 sends status information to the base unit 112 at regular intervals. In one embodiment, the controller 303 sends status information to the base unit 112 when requested by the base unit 112. In one embodiment, the controller 303 sends status information to the base unit 112 when a fault condition (e.g., battery low) is detected.

In one embodiment, the controller 303 includes a table or list of identification codes for wireless sensor units 102. The repeater 303 forwards packets received from, or sent to, sensor units 102 in the list. In one embodiment, the repeater 110 receives entries for the list of sensor units from the computer 113. In one embodiment, the controller 303 determines when a transmission is expected from the sensor units 102 in the table of sensor units and places the repeater 110 (e.g., the transceivers 302, 304) in a low-power mode when no transmissions are expected from the transceivers on the list. In one embodiment, the controller 303 recalculates the times for low-power operation when a command to change reporting interval is forwarded to one of the sensor units 102 in the list (table) of sensor units or when a new sensor unit is added to the list (table) of sensor units.

FIG. 4 is a block diagram of the base unit 112. In the base unit 112, a transceiver 402 and a computer interface 404 are provided to a controller 403. The controller 303 typically provides data and control information to the transceivers 402 and to the interface. The interface 402 is provided to a port on the monitoring computer 113. The interface 402 can be a standard computer data interface, such as, for example, Ethernet, wireless Ethernet, firewire port, Universal Serial Bus (USB) port, bluetooth, etc.

FIG. 5 shows one embodiment a communication packet 500 used by the sensor units, repeater units, and the base unit. The packet 500 includes a preamble portion 501, an address (or ID) portion 502, a data payload portion 503, and an integrity portion 504. In one embodiment, the integrity portion 504 includes a checksum. In one embodiment, the sensor units 102-106, the repeater units 110-111, and the base unit 112 communicate using packets such as the packet 500. In one embodiment, the packets 500 are transmitted using FHSS.

In one embodiment, the data packets that travel between the sensor unit 102, the repeater unit 111, and the base unit 112 are encrypted. In one embodiment, the data packets that travel between the sensor unit 102, the repeater unit 111, and the base unit 112 are encrypted and an authentication code is provided in the data packet so that the sensor unit 102, the repeater unit, and/or the base unit 112 can verify the authenticity of the packet.

In one embodiment the address portion 502 includes a first code and a second code. In one embodiment, the repeater 111 only examines the first code to determine if the packet should be forwarded. Thus, for example, the first code can be interpreted as a building (or building complex) code and the second code interpreted as a subcode (e.g., an apartment code, area code, etc.). A repeater that uses the first code for forwarding thus forwards packets having a specified first code (e.g., corresponding to the repeater's building or building complex). Thus alleviates the need to program a list of sensor units 102 into a repeater, since a group of sensors in a building will typically all have the same first code but different second codes. A repeater so configured, only needs to know the first code to forward packets for any repeater in the building or building complex. This does, however, raise the possibility that two repeaters in the same building could try to forward packets for the same sensor unit 102. In one embodiment, each repeater waits for a programmed delay period before forwarding a packet. Thus reducing the chance of packet collisions at the base unit (in the case of sensor unit to base unit packets) and reducing the chance of packet collisions at the sensor unit (in the case of base unit to sensor unit packets). In one embodiment, a delay period is programmed into each repeater. In one embodiment, delay periods are pre-programmed onto the repeater units at the factory or during installation. In one embodiment, a delay period is programmed into each repeater by the base unit 112. In one embodiment, a repeater randomly chooses a delay period. In one embodiment, a repeater randomly chooses a delay period for each forwarded packet. In one embodiment, the first code is at least 6 digits. In one embodiment, the second code is at least 5 digits.

In one embodiment, the first code and the second code are programmed into each sensor unit at the factory. In one embodiment, the first code and the second code are programmed when the sensor unit is installed. In one embodiment, the base unit 112 can re-program the first code and/or the second code in a sensor unit.

In one embodiment, collisions are further avoided by configuring each repeater unit 111 to begin transmission on a different frequency channel. Thus, if two repeaters attempt to begin transmission at the same time, the repeaters will not interfere with each other because the transmissions will begin on different channels (frequencies).

FIG. 6 is a flowchart showing one embodiment of the operation of the sensor unit 102 wherein relatively continuous monitoring is provided. In FIG. 6, a power up block 601 is followed by an initialization block 602. After initialization, the sensor unit 102 checks for a fault condition (e.g., activation of the tamper sensor, low battery, internal fault, etc.) in a block 603. A decision block 604 checks the fault status. If a fault has occurred, then the process advances to a block 605 were the fault information is transmitted to the repeater 110 (after which, the process advances to a block 612); otherwise, the process advances to a block 606. In the block 606, the sensor unit 102 takes a sensor reading from the sensor(s) 201. The sensor data is subsequently evaluated in a block 607. If the sensor data is abnormal, then the process advances to a transmit block 609 where the sensor data is transmitted to the repeater 110 (after which, the process advances to a block 612); otherwise, the process advances to a timeout decision block 610. If the timeout period has not elapsed, then the process returns to the fault-check block 603; otherwise, the process advances to a transmit status block 611 where normal status information is transmitted to the repeater 110. In one embodiment, the normal status information transmitted is analogous to a simple “ping” which indicates that the sensor unit 102 is functioning normally. After the block 611, the process proceeds to a block 612 where the sensor unit 102 momentarily listens for instructions from the monitor computer 113. If an instruction is received, then the sensor unit 102 performs the instructions, otherwise, the process returns to the status check block 603. In one embodiment, transceiver 203 is normally powered down. The controller 202 powers up the transceiver 203 during execution of the blocks 605, 609, 611, and 612. The monitoring computer 113 can send instructions to the sensor unit 102 to change the parameters used to evaluate data used in block 607, the listen period used in block 612, etc.

Relatively continuous monitoring, such as shown in FIG. 6, is appropriate for sensor units that sense relatively high-priority data (e.g., smoke, fire, carbon monoxide, flammable gas, etc.). By contrast, periodic monitoring can be used for sensors that sense relatively lower priority data (e.g., humidity, moisture, water usage, etc.). FIG. 7 is a flowchart showing one embodiment of operation of the sensor unit 102 wherein periodic monitoring is provided. In FIG. 7, a power up block 701 is followed by an initialization block 702. After initialization, the sensor unit 102 enters a low-power sleep mode. If a fault occurs during the sleep mode (e.g., the tamper sensor is activated), then the process enters a wake-up block 704 followed by a transmit fault block 705. If no fault occurs during the sleep period, then when the specified sleep period has expired, the process enters a block 706 where the sensor unit 102 takes a sensor reading from the sensor(s) 201. The sensor data is subsequently sent to the monitoring computer 113 in a report block 707. After reporting, the sensor unit 102 enters a listen block 708 where the sensor unit 102 listens for a relatively short period of time for instructions from monitoring computer 708. If an instruction is received, then the sensor unit 102 performs the instructions, otherwise, the process returns to the sleep block 703. In one embodiment, the sensor 201 and transceiver 203 are normally powered down. The controller 202 powers up the sensor 201 during execution of the block 706. The controller 202 powers up the transceiver during execution of the blocks 705, 707, and 708. The monitoring computer 113 can send instructions to the sensor unit 102 to change the sleep period used in block 703, the listen period used in block 708, etc.

In one embodiment, the sensor unit transmits sensor data until a handshaking-type acknowledgement is received. Thus, rather than sleep of no instructions or acknowledgements are received after transmission (e.g., after the decision block 613 or 709) the sensor unit 102 retransmits its data and waits for an acknowledgement. The sensor unit 102 continues to transmit data and wait for an acknowledgement until an acknowledgement is received. In one embodiment, the sensor unit accepts an acknowledgement from a repeater unit 111 and it then becomes the responsibility of the repeater unit 111 to make sure that the data is forwarded to the base unit 112. In one embodiment, the repeater unit 111 does not generate the acknowledgement, but rather forwards an acknowledgement from the base unit 112 to the sensor unit 102. The two-way communication ability of the sensor unit 102 provides the capability for the base unit 112 to control the operation of the sensor unit 102 and also provides the capability for robust handshaking-type communication between the sensor unit 102 and the base unit 112.

Regardless of the normal operating mode of the sensor unit 102 (e.g., using the Flowcharts of FIGS. 6, 7, or other modes) in one embodiment, the monitoring computer 113 can instruct the sensor unit 102 to operate in a relatively continuous mode where the sensor repeatedly takes sensor readings and transmits the readings to the monitoring computer 113. Such a mode can be used, for example, when the sensor unit 102 (or a nearby sensor unit) has detected a potentially dangerous condition (e.g., smoke, rapid temperature rise, etc.).

FIG. 8 shows the sensor system used to detect water leaks. In one embodiment, the sensor unit 102 includes a water level sensor and 803 and/or a water temperature sensor 804. The water level sensor 803 and/or water temperature sensor 804 are place, for example, in a tray underneath a water heater 801 in order to detect leaks from the water heater 801 and thereby prevent water damage from a leaking water heater. In one embodiment, a temperature sensor is also provide to measure temperature near the water heater. The water level sensor can also be placed under a sink, in a floor sump, etc. In one embodiment, the severity of a leak is ascertained by the sensor unit 102 (or the monitoring computer 113) by measuring the rate of rise in the water level. When placed near the hot water tank 801, the severity of a leak can also be ascertained at least in part by measuring the temperature of the water. In one embodiment, a first water flow sensor is placed in an input water line for the hot water tank 801 and a second water flow sensor is placed in an output water line for the hot water tank. Leaks in the tank can be detected by observing a difference between the water flowing through the two sensors.

In one embodiment, a remote shutoff valve 810 is provided, so that the monitoring system 100 can shutoff the water supply to the water heater when a leak is detected. In one embodiment, the shutoff valve is controlled by the sensor unit 102. In one embodiment, the sensor unit 102 receives instructions from the base unit 112 to shut off the water supply to the heater 801. In one embodiment, the responsible party 120 sends instructions to the monitoring computer 113 instructing the monitoring computer 113 to send water shut off instructions to the sensor unit 102. Similarly, in one embodiment, the sensor unit 102 controls a gas shutoff valve 811 to shut off the gas supply to the water heater 801 and/or to a furnace (not shown) when dangerous conditions (such as, for example, gas leaks, carbon monoxide, etc.) are detected. In one embodiment, a gas detector 812 is provided to the sensor unit 102. In one embodiment, the gas detector 812 measures carbon monoxide. In one embodiment, the gas detector 812 measures flammable gas, such as, for example, natural gas or propane.

In one embodiment, an optional temperature sensor 818 is provided to measure stack temperature. Using data from the temperature sensor 818, the sensor unit 102 reports conditions, such as, for example, excess stack temperature. Excess stack temperature is often indicative of poor heat transfer (and thus poor efficiency) in the water heater 818.

In one embodiment, an optional temperature sensor 819 is provided to measure temperature of water in the water heater 810. Using data from the temperature sensor 819, the sensor unit 102 reports conditions, such as, for example, over-temperature or under-temperature of the water in the water heater.

In one embodiment, an optional current probe 821 is provided to measure electric current provided to a heating element 820 in an electric water heater. Using data from the current probe 821, the sensor unit 102 reports conditions, such as, for example, no current (indicating a burned-out heating element 820). An over-current condition often indicates that the heating element 820 is encrusted with mineral deposits and needs to be replaced or cleaned. By measuring the current provided to the water heater, the monitoring system can measure the amount of energy provided to the water heater and thus the cost of hot water, and the efficiency of the water heater.

In one embodiment, the sensor 803 includes a moisture sensor. Using data from the moisture sensor, the sensor unit 102 reports moisture conditions, such as, for example, excess moisture that would indicate a water leak, excess condensation, etc.

In one embodiment, the sensor unit 102 is provided to a moisture sensor (such as the sensor 803) located near an air conditioning unit. Using data from the moisture sensor, the sensor unit 102 reports moisture conditions, such as, for example, excess moisture that would indicate a water leak, excess condensation, etc.

In one embodiment, the sensor 201 includes a moisture sensor. The moisture sensor can be place under a sink or a toilet (to detect plumbing leaks) or in an attic space (to detect roof leaks).

Excess humidity in a structure can cause sever problems such as rotting, growth of molds, mildew, and fungus, etc. (hereinafter referred to generically as fungus). In one embodiment, the sensor 201 includes a humidity sensor. The humidity sensor can be place under a sink, in an attic space, etc. to detect excess humidity (due to leaks, condensation, etc.). In one embodiment, the monitoring computer 113 compares humidity measurements taken from different sensor units in order to detect areas that have excess humidity. Thus for example, the monitoring computer 113 can compare the humidity readings from a first sensor unit 102 in a first attic area, to a humidity reading from a second sensor unit 102 in a second area. For example, the monitoring computer can take humidity readings from a number of attic areas to establish a baseline humidity reading and then compare the specific humidity readings from various sensor units to determine if one or more of the units are measuring excess humidity. The monitoring computer 113 would flag areas of excess humidity for further investigation by maintenance personnel. In one embodiment, the monitoring computer 113 maintains a history of humidity readings for various sensor units and flags areas that show an unexpected increase in humidity for investigation by maintenance personnel.

In one embodiment, the monitoring system 100 detects conditions favorable for fungus (e.g., mold, mildew, fungus, etc.) growth by using a first humidity sensor located in a first building area to produce first humidity data and a second humidity sensor located in a second building area to produce second humidity data. The building areas can be, for example, areas near a sink drain, plumbing fixture, plumbing, attic areas, outer walls, a bilge area in a boat, etc.

The monitoring station 113 collects humidity readings from the first humidity sensor and the second humidity sensor and indicates conditions favorable for fungus growth by comparing the first humidity data and the second humidity data. In one embodiment, the monitoring station 113 establishes a baseline humidity by comparing humidity readings from a plurality of humidity sensors and indicates possible fungus growth conditions in the first building area when at least a portion of the first humidity data exceeds the baseline humidity by a specified amount. In one embodiment, the monitoring station 113 establishes a baseline humidity by comparing humidity readings from a plurality of humidity sensors and indicates possible fungus growth conditions in the first building area when at least a portion of the first humidity data exceeds the baseline humidity by a specified percentage.

In one embodiment, the monitoring station 113 establishes a baseline humidity history by comparing humidity readings from a plurality of humidity sensors and indicates possible fungus growth conditions in the first building area when at least a portion of the first humidity data exceeds the baseline humidity history by a specified amount over a specified period of time. In one embodiment, the monitoring station 113 establishes a baseline humidity history by comparing humidity readings from a plurality of humidity sensors over a period of time and indicates possible fungus growth conditions in the first building area when at least a portion of the first humidity data exceeds the baseline humidity by a specified percentage of a specified period of time.

In one embodiment, the sensor unit 102 transmits humidity data when it determines that the humidity data fails a threshold test. In one embodiment, the humidity threshold for the threshold test is provided to the sensor unit 102 by the monitoring station 113. In one embodiment, the humidity threshold for the threshold test is computed by the monitoring station from a baseline humidity established in the monitoring station. In one embodiment, the baseline humidity is computed at least in part as an average of humidity readings from a number of humidity sensors. In one embodiment, the baseline humidity is computed at least in part as a time average of humidity readings from a number of humidity sensors. In one embodiment, the baseline humidity is computed at least in part as a time average of humidity readings from a humidity sensor. In one embodiment, the baseline humidity is computed at least in part as the lesser of a maximum humidity reading an average of a number of humidity readings.

In one embodiment, the sensor unit 102 reports humidity readings in response to a query by the monitoring station 113. In one embodiment, the sensor unit 102 reports humidity readings at regular intervals. In one embodiment, a humidity interval is provided to the sensor unit 102 by the monitoring station 113.

In one embodiment, the calculation of conditions for fungus growth is comparing humidity readings from one or more humidity sensors to the baseline (or reference) humidity. In one embodiment, the comparison is based on comparing the humidity readings to a percentage (e.g., typically a percentage greater than 100%) of the baseline value. In one embodiment, the comparison is based on comparing the humidity readings to a specified delta value above the reference humidity. In one embodiment, the calculation of likelihood of conditions for fungus growth is based on a time history of humidity readings, such that the longer the favorable conditions exist, the greater the likelihood of fungus growth. In one embodiment, relatively high humidity readings over a period of time indicate a higher likelihood of fungus growth than relatively high humidity readings for short periods of time. In one embodiment, a relatively sudden increase in humidity as compared to a baseline or reference humidity is reported by the monitoring station 113 as a possibility of a water leak. If the relatively high humidity reading continues over time then the relatively high humidity is reported by the monitoring station 113 as possibly being a water leak and/or an area likely to have fungus growth or water damage.

Temperatures relatively more favorable to fungus growth increase the likelihood of fungus growth. In one embodiment, temperature measurements from the building areas are also used in the fungus grown-likelihood calculations. In one embodiment, a threshold value for likelihood of fungus growth is computed at least in part as a function of temperature, such that temperatures relatively more favorable to fungus growth result in a relatively lower threshold than temperatures relatively less favorable for fungus growth. In one embodiment, the calculation of a likelihood of fungus growth depends at least in part on temperature such that temperatures relatively more favorable to fungus growth indicate a relatively higher likelihood of fungus growth than temperatures relatively less favorable for fungus growth. Thus, in one embodiment, a maximum humidity and/or minimum threshold above a reference humidity is relatively lower for temperature more favorable to fungus growth than the maximum humidity and/or minimum threshold above a reference humidity for temperatures relatively less favorable to fungus growth.

In one embodiment, a water flow sensor is provided to the sensor unit 102. The sensor unit 102 obtains water flow data from the water flow sensor and provides the water flow data to the monitoring computer 113. The monitoring computer 113 can then calculate water usage. Additionally, the monitoring computer can watch for water leaks, by, for example, looking for water flow when there should be little or no flow. Thus, for example, if the monitoring computer detects water usage throughout the night, the monitoring computer can raise an alert indicating that a possible water leak has occurred.

In one embodiment, the sensor 201 includes a water flow sensor is provided to the sensor unit 102. The sensor unit 102 obtains water flow data from the water flow sensor and provides the water flow data to the monitoring computer 113. The monitoring computer 113 can then calculate water usage. Additionally, the monitoring computer can watch for water leaks, by, for example, looking for water flow when there should be little or no flow. Thus, for example, if the monitoring computer detects water usage throughout the night, the monitoring computer can raise an alert indicating that a possible water leak has occurred.

In one embodiment, the sensor 201 includes a fire-extinguisher tamper sensor is provided to the sensor unit 102. The fire-extinguisher tamper sensor reports tampering with or use of a fire-extinguisher. In one embodiment the fire-extinguisher tamper sensor reports that the fire extinguisher has been removed from its mounting, that a fire extinguisher compartment has been opened, and/or that a safety lock on the fire extinguisher has been removed.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributed thereof; furthermore, various omissions, substitutions and changes may be made without departing from the spirit of the inventions. For example, although specific embodiments are described in terms of the 900 MHz frequency band, one of ordinary skill in the art will recognize that frequency bands above and below 900 MHz can be used as well. The wireless system can be configured to operate on one or more frequency bands, such as, for example, the HF band, the VHF band, the UHF band, the Microwave band, the Millimeter wave band, etc. One of ordinary skill in the art will further recognize that techniques other than spread spectrum can also be used and/or can be use instead spread spectrum. The modulation uses is not limited to any particular modulation method, such that modulation scheme used can be, for example, frequency modulation, phase modulation, amplitude modulation, combinations thereof, etc. The foregoing description of the embodiments is therefore to be considered in all respects as illustrative and not restrictive, with the scope of the invention being delineated by the appended claims and their equivalents. 

What is claimed is:
 1. A method of avoiding transmission collisions within a wireless sensor system, comprising: programming a delay value into each of one or more wireless repeater units; waiting for said delay value by each of said one or more wireless repeater units before forwarding communications from a base unit to one or more wireless sensor units; and waiting for said delay value by each of said one or more wireless repeater units before forwarding communications from said one or more wireless sensor units to said base unit.
 2. The method of avoiding transmission collisions within a wireless sensor system of claim 1, wherein the delay value is programmed into the one or more wireless repeater units by the base unit.
 3. The method of avoiding transmission collisions within a wireless sensor system of claim 1, wherein the delay value for the one or more wireless repeater units is randomly chosen by each of the one or more wireless repeater units.
 4. The method of avoiding transmission collisions within a wireless sensor system of claim 1, wherein the one or more wireless repeater units each randomly chooses a delay value for each forwarded packet of said communications.
 5. The method of avoiding transmission collisions within a wireless sensor system of claim 1, wherein each of said one or more wireless repeater units is configured to begin transmission on different frequency channels, such that if two repeaters attempt to begin transmission at the same time, the transmissions will begin on different channels.
 6. The method of avoiding transmission collisions within a wireless sensor system of claim 1, further comprising: forwarding communications from the base unit to the one or more wireless sensor units on a first frequency channel; and forwarding communications from said one or more wireless sensor units to said base unit on a different frequency channel, such that if two repeaters attempt to begin transmission at the same time, the transmissions will begin on different channels.
 7. The method of avoiding transmission collisions within a wireless sensor system of claim 1, wherein each of said one or more wireless repeater units is configured to listen to a frequency channel, wherein the waiting for said delay value by each of said one or more wireless repeater units is in response to a determination that there is activity on the frequency channel.
 8. The method of avoiding transmission collisions within a wireless sensor system of claim 7, wherein the delay value for the one or more wireless repeater units is randomly chosen by each of the one or more wireless repeater units and the delay value for a first one of the one or more wireless repeater units is different than the delay value for a second one of the one or more wireless repeater units.
 9. A wireless repeater unit, comprising: a wireless transceiver capable of wirelessly transmitting and receiving information to and from electronic devices located remotely from the wireless repeater unit; a power source for powering operation of the wireless repeater unit; and a controller operatively coupled to the wireless transceiver and the power source and configured to: program a delay value into the wireless repeater unit; cause the wireless repeater unit to wait for said delay value before forwarding communications, via the wireless transceiver, from a base unit to one or more wireless sensor units; and cause the wireless repeater unit to wait for said delay value before forwarding communications, via the wireless transceiver, from said one or more wireless sensor units to said base unit.
 10. The wireless repeater unit of claim 9, further comprising memory wherein the controller is configured to program a delay value by storing a randomly-chosen delay value into the memory of the wireless repeater unit.
 11. The wireless repeater unit of claim 9, wherein the controller is configured to program the delay value by wirelessly receiving the delay value via the wireless transceiver.
 12. The wireless repeater unit of claim 9, further comprising a list of sensor units, wherein the controller is configured to cause the wireless repeater unit to forward communications from the base unit to one or more wireless sensor units only when the one or more wireless sensor units are on the list of sensor units.
 13. The wireless repeater unit of claim 9, further comprising a unique identifier associated with the wireless repeater unit for uniquely identifying the wireless repeater unit within a network, wherein the controller is further configured to: receive a communication from the base unit; determine whether the communication includes the unique identifier; when it is determined that the communication includes the unique identifier, forward the communication to the one or more wireless sensor units after waiting for the delay value; and when it is determined that the communication does not include the unique identifier, ignore the communication.
 14. The wireless repeater unit of claim 13, wherein the communication includes a command for instructing at least one of the one or more wireless sensor units to perform an operation.
 15. The wireless repeater unit of claim 14, wherein the command is selected from the group consisting of: wake up, report status, and change wake-up interval.
 16. A method of avoiding transmission collisions within a wireless sensor system, comprising: determining at least one delay value for a wireless repeater unit; waiting for a first delay value of the at least one delay value before forwarding communications from a base unit to one or more wireless sensor units via the wireless repeater unit; and waiting for a second delay value of the at least one delay value before forwarding communications from said one or more wireless sensor units to said base unit via the wireless repeater unit.
 17. The method of avoiding transmission collisions within a wireless sensor system of claim 16, wherein each of the at least one delay value is a random value, the first delay value is a random value, the second delay value is a random value, and the first delay value is different than the second delay value.
 18. The method of avoiding transmission collisions within a wireless sensor system of claim 16, wherein the communications forwarded from the base unit to the one or more wireless sensor units are sent by the wireless repeater unit until an acknowledgement message is received by the wireless repeater unit from the one or more wireless sensor units.
 19. The method of avoiding transmission collisions within a wireless sensor system of claim 16, further comprising: receiving, at the wireless repeater unit and from the one or more wireless sensor units for forwarding to the base unit, communications at programmed intervals; and receiving, at the wireless repeater unit and from the one or more wireless sensor units for forwarding to the base unit, communications at intermittent periods of time that are representative of a dangerous condition, the dangerous condition indicating that an ambient environmental condition exceeds at least one threshold of at least one sensor included in the one or more wireless sensor units.
 20. The method of avoiding transmission collisions within a wireless sensor system of claim 16, wherein the communications received at intermittent periods of time that are representative of a dangerous condition include a message indicative of an ambient environmental condition exceeding a threshold of a sensor included in one of the one or more wireless sensor units, a unique identifier that uniquely identifies the one of the one or more wireless sensor units within the wireless sensor system, and an integrity portion for verifying the integrity of the message, and the method further comprises forwarding the message and the unique identifier to the base unit after waiting for the second delay value. 